1. Field of the Invention
The present invention relates to a technique of printing an image on a printing medium, and more specifically to a printing technique that records two pixels adjoining to each other in a main scanning direction with a plurality of ink droplets.
2. Description of the Related Art
Ink jet printers that eject ink droplets from a head are widely used as an output device of a computer. The conventional ink jet printers reproduce each pixel by only two values, that is, the on state and the off state. Multi-value printers, which have been proposed recently, on the other hand, reproduce each pixel by three or greater values.
One of such multi-value printers selectively ejects a first ink droplet, which has a relatively small quantity of ink, and a second ink droplet, which has a greater quantity of ink than that of the first ink droplet, in the area of one pixel. This configuration enables reproduction of four tones, that is, the state of no dot creation where neither the first ink droplet nor the second ink droplet is ejected, the state of small dot creation where only the first ink droplet is ejected, the state of medium dot creation where only the second ink droplet is ejected, and the state of large dot creation where both the first ink droplet and the second ink droplet are ejected. The arrangement of ejecting the two different types of ink droplets is actualized by driving the print head in response to a driving signal, which may selectively include a first driving pulse and a second driving pulse within one printing period corresponding to one pixel in printing.
In the prior art technique, there are some cases in which two different types of dots are created respectively in two pixels adjoining to each other in the main scanning direction in response to different driving pulses selected out of the first and the second driving pulses. The positions of these two adjoining dots in the main scanning direction created by the prior art technique are, however, varied to cause a positional deviation. Namely there is a difference between a first state, in which a dot is created in the first pixel in response to the first driving pulse and a dot is created in the latter pixel in response to the second driving pixel, and a second state, in which a dot is created in the first pixel in response to the second driving pulse and a dot is created in the latter pixel in response to the first driving pixel. The subsequent image processing does not practically distinguish between the first state and the second state. The prior art technique accordingly fails in faithful reproduction of print data of interest generated as a result of the image processing, which causes deterioration of the picture quality of the resulting printed image.
FIG. 25 shows the positions of two different types of dots, a small dot and a medium dot, created in the first state and in the second state. Lattices in FIG. 25 represent boundaries of pixel areas, and each square area defined by a lattice corresponds to the area of one pixel. An ink droplet is ejected from a print head (not shown) into each pixel, while the print head moves in the main scanning direction. In the example of FIG. 25, recording is carried out in the first state with regard to two pixels, a k-th pixel and a (k+1)-th pixel (where k is a positive number), that are included in a first raster line L1 and adjoin to each other in the main scanning direction. Recording is carried out in the second state, on the other hand, with regard to two pixels, a k-th pixel and a (k+1)-th pixel, that are included in a second raster line L2 and adjoin to each other in the main scanning direction.
As clearly understood from the drawing of FIG. 25, in the prior art technique, the hitting positions of the two ink droplets ejected in the two adjoining pixels, the k-th and (k+1)-th pixels, on the first raster line L1 are different from those on the second raster line L2. The ink droplet for recording the k-th pixel in the main scanning direction hits on the left half of the pixel area in the first raster line L1, but hits on the right half of the pixel area in the second raster line L2. On the contrary, the ink droplet for recording the (k+1)-th pixel hits on the right half of the pixel area in the first raster line L1, but hits on the left half of the pixel area in the second raster line L2. The subsequent image processing does not distinguish between the two dots on the first raster line L1 and the two dots on the second raster line L2. The small dot is, however, apart from the medium dot on the first raster line L1, whereas the medium dot is close to or even integrated with the small dot on the second raster line L2. This results in a density difference and roughness in the resulting reproduced image.
In the conventional multi-value ink jet printer, the hitting positions of the two different types of ink droplets in the main scanning direction, which are ejected in the two adjoining pixels, are varied in the first state and in the second state discussed above. The variation in hitting positions unfavorably deteriorates the picture quality of the resulting printed image.